A preferred application of the invention is the production of inlet valves for diesel engines, referred to as “EGR” types (exhaust gases recycling), in which these valves are placed in contact with combustion gases which are reinjected at the inlet in order to ensure the complete combustion thereof and the reduction of the quantity of polluting discharged products.
When they are used, these valves can locally reach temperatures of from 400 to 500° C. and are subjected to high levels of mechanical stress and a corrosive environment. Furthermore, the condensates which are deposited on the valves during a prolonged idle period of this type of engine are also very aggressive.
Under these aggressive conditions, the grades of carbon steels, low-alloyed steels and chromium- and silicon-alloyed steels which are conventionally used to produce valves have insufficient resistance to corrosion.
Austenitic steels which are highly alloyed offer a very good level of resistance to corrosion under these conditions but their material cost is significantly higher.
Furthermore, these austenitic steels cannot be hardened by means of quenching. The contacting surface of the valve and the end of the stem thereof must have high levels of hardness and good resistance to abrasion. Generally, valves of austenitic steel are therefore constituted by from two to three components, including:                the valve seat contacting surface, which is produced from an alloy which has a high level of hardness, which is resistant to corrosion, and which is deposited by means of coating,        the coldest end of the stem, which is produced from a martensitic steel which is capable of high levels of hardness by means of quenching.        
In all cases, this increases the cost of the valve.
Furthermore, the high creep resistance of austenitic steels is not advantageous at temperatures less than or equal to 500° C. Finally, the lower thermal conductivity of austenitic steels is rather disadvantageous in practice.
The use of austenitic steels therefore constitutes “excessive quality” in some respects and also has technical disadvantages. Furthermore, the high cost of implementing this solution makes it on the whole unsatisfactory at an industrial level.
There is therefore a need on the part of motorists to have a method which allows inlet valves to be produced for EGR diesel engines which have high levels of mechanical property during operation and a high level of resistance to corrosion, whilst retaining a low production cost, in particular since it would be possible to produce this valve in the form of a single solid component.